Crack growth analyses in multi-layered architectures used in the automotive industry for plain bearings, displaying elastic and plastic mismatches, were developed through automated meshing processes based on the Finite Element Method in the commercial code ANSYS. Two-dimensional FE analyses studied the effect of shielding and anti-shielding on crack growth in flat strip specimens subjected to three-point bending tests experiencing severe yielding and path deflections. This study was based on Crack Tip Opening Displacement estimates, the maximum tangential strain criterion and automatic step by step extensions to account for crack growth. The analysis of forecasted paths for deflected and bifurcated cracks showed the tendency of the crack to grow parallel to the layers orientation within the compliant interlayer, as observed experimentally, and following a path which maximised the crack driving force. The interaction of two-dimensional width-through cracks in co-linear, parallel-dominant and oblique arrangements was studied to compare their behaviour to independent growing cracks. The development of three-dimensional models was aimed to study the crack front evolution when more compliant or stiffer layers were placed ahead of the crack in flat strip specimens subjected to three-point bending tests experiencing severe yielding. The crack front growth was estimated according to the local values of the Crack Tip Opening Displacement and a fitted crack growth law, which accounted for the short fatigue crack behaviour observed experimentally. The comparison of Crack Tip Opening Displacements obtained with two-dimensional and three-dimensional models showed that the estimates obtained with the latter were affected to a lesser extent by shielding and anti-shielding. The estimation of cycles to propagate a crack to the interlayer, in addition to the initiation life obtained in previous experimental work at the University of Southampton, estimated that less than 40% of the total life was required for such a process and demonstrating the damage tolerance of the layered architecture. A detailed stress and strain analysis of the service conditions, based on the Elasto-Hydrodynamic Lubrication pressure, and manufacturing processes, mainly related to the forming of a concave shell, was developed for plain bearings as the initial step of a damage tolerance analysis.

We have performed direct numerical simulations (DNS) of quasi-2D (that is with flow parameters independent of longitudinal coordinate) decaying and forced turbulence and 3D turbulent channel flows in order to ascertain the sustenance mechanism of near-wall turbulence by investigating the mechanism of streak formation. We found the existence of streaks in quasi-2D flows thus demonstrating that contrary to many proposed theories, feedback from longitudinal flow is not necessary for streak formation. Passive scalars having different mean scalar profiles were introduced in forced quasi-2D and 3D turbulent flows in order to compare the streak spacing of the scalars deduced from two-point correlations of DNS results with results obtained theoretically. It has been found that even for the same vortex structure for all the passive scalars there is a marked variation in streak spacing implying that the preferential streak spacing is not necessarily equal to twice the vortex spacing, as has been suggested by several proposed theories. Moreover, the formation of scalar streaks in a velocity field prescribed as s sum of a mean turbulent velocity profile and random potential perturbations, conclusively supports the fact that organised vortices are not needed for generation of near-wall streaks. It has also been demonstrated that the lift-up mechanism responsible for generation of streaks is also responsible for the cross-flow spacing. The obtained qualitative numerical results are in favour of theory of streak formation based on optimal perturbations (Butler and Farrell, 1993) but at the same time the quantitative agreement is poor. So a modification of the same - Generalized optimal perturbation (Chernyshenko and Baig, 2003) theory has been proposed and it offers significantly better agreement with the DNS results.

Small automotive plain engine bearings are used to provide the relative motion between the engine block and the crankshaft via the connecting rod. Under rapidly changing engine loads, these bearings may suffer fatigue damage during service. In modern multilayered bearing designs, fatigue resistance is a complex function of engine loading coupled with the layer architecture and a multiphase lining alloy. This research has mostly focussed upon micro-scale fatigue damage initiation on thin (0.2-0.3mm thickness) lining surface and its subsequent growth leading towards gross failure. The systems examined comprise Al alloys and sintered bronze as relatively soft and conformable lining layers. The weight percent composition of Al lining alloy was Al-6.5Sn-2.5Si-1Cu-1Ni-0.25Mn roll bonded to a stiffer and thicker backing steel layer (1.5-1.8mm thick) via an even thinner Al foil (0.04mm) as an interlayer. The other system comprised an Al lining (Al-20Sn–1Cu) alloy spray coated on to a medium carbon steel layer in the form of a flat bar. All these systems were compared with the previously investigated Al based designs with lining compositions: Al-12Sn-4Si-1Cu and Al-20Sn-1Cu-0.25Mn (manufactured by roll bonding processes). The performance evaluation was based upon the investigation of microstructural features involved in early fatigue initiation and their effect upon short crack growth on the surface. Subsurface crack growth through the layers has also been assessed and finally the observed fatigue life of various components linked to these behaviours. A 3-point bend test configuration was adopted for laboratory fatigue tests. Fatigue comparison was made on the basis of lining surface plastic strain amplitude vs. number of cycles to failure according to a uniform predefined criterion for all the systems. Maximum plastic strains developing at the lining surface were estimated using a combination of finite element analysis (FEA) and strain gauge measurements so that the fatigue life of all systems studied was presented as strain-life data. Specimens in the form of both finished bearings and flat bars were tested. Similar fatigue behaviour was observed for the two testing geometries, giving greater confidence in the fatigue evaluation process and allowing detailed observations of small crack initiation and growth processes in flat bars to be related to behaviour of the actual bearing geometry. In the previous research, the coarser Si particles in the Al-12Sn-4Si-1Cu lining and Sn particles in the Al-20Sn-1Cu-0.25Mn alloys were identified as potential crack initiation sites, though the relationship between particle geometry and arrangement/clustering was found to be important. The newly developed Al-6.5Sn-2.5Si-1Cu-1Ni-0.25Mn lining alloy with finer and fewer Sn and Si particles showed a delayed initiation of short fatigue cracks compared to the previous systems. However, a large number of widely scattered intermetallics in the new linings were observed to fracture causing early fatigue initiation at the micro-scale level with some more complex processes of detaching Sn layers from harder intermetallics and Si particles. Using the mechanical property data for bulk lining and secondary phase particles obtained from tensile testing and instrumented hardness testing, stress fields were investigated within the hard particles (intermetallics), surrounding thin layers and the matrix on the basis of the analytical and numerical modelling. On the basis of these modelling results, optimum particle shapes were defined to minimize tensile stresses (within the particles) and hydrostatic stresses (at the particle matrix interfaces). The experimental growth data of a dominant crack when combined with a Hobson type growth model based upon measured particle distributions and experimental crack growth rates, helped in predicting fatigue life of a similar component at different stress levels. Surface crack driving force reduces considerably when subsurface crack deflection occurred within softer Al interlayer. Replacing this interlayer with a harder brazed sheet did not give any significant difference in the observed fatigue life. In the HVOF systems, crack initiation was observed to be from the weaker interface between a harder matrix and softer circular unmelts as well as from various scattered pores. The overall fatigue life of the HVOF systems was comparable to the previous roll bonded systems; however subsurface deflection of crack at the lining-backing interface resulted in the debonding of the lining and hence the observed lining fatigue resistance may not be a good indication of the overall performance in a bearing system. At similar lining surface plastic strain levels, the bronze bearing with very thin Sn and Ni as overlay layers (~7 microns each) showed comparable fatigue resistance to the currently investigated RB Al based designs. However annealing this system resulted in the formation of hard Ni3Sn intermetallics at the Sn-Ni interface, and the observed fatigue resistance of this system was higher than the RB systems. This has been linked to very fine scale local crack deflection in the overlay layers (although these have not been observed clearly). All these layered bearing systems provide a complex fatigue problem. Factors which reduce initiation /early growth behaviour are likely to offer the best service performance enhancements in view of the relatively HCF nature expected in service.

No description available.

No description available.

The expense of high fidelity computational fluid dynamics, in terms of time and amount of computing resources required, excludes such methods from the early stages of aircraft design. It is only in the early, conceptual, stage of aircraft development where a wide range of designs are considered and global, rather than local, optimisation can play a key role. This thesis deals with methods which may allow high cost computer simulations to be used within a global optimisation design process. The first half of the thesis concentrates on the use of surrogate modeling of the optimisation design space, which allows cheap approximations to be used in lieu of expensive computer simulations. The process is automated and present statistical methods are modified to accommodate problems associated with the simulation of fluid flow and uncertainty within an automated system. The re-interpolation of a regression model of noisy data is presented as a method of improving convergence towards a global optimum. The second half of the thesis develops methods of using partially converged computational fluid dynamics simulations within a surrogate modelling optimisation process. Significant time savings are made possible by reducing computational effort directed at producing a surrogate for regions of poor designs and concentrating resources on modelling regions of promising designs.

This thesis is concerned with a theoretical investigation into the nonlinear dynamic behaviour of a turbocharger. Specifically the instabilities due to oil-whirl are examined. These are self-excited vibrations existing in the form of an in-phase whirl mode and a conical whirl mode. Waterfall plots were provided by Cummins Turbo-Technologies Ltd., Huddersfield, UK, based on test data using two different unbalance levels on a turbocharger. The test with the high unbalance indicated that there was shift in the sub-synchronous frequency to synchronous frequency between about 80,000 rpm to 130,000 rpm. The literature suggests that this self-excited vibration can be suppressed using forced excitation. Moreover, it is well known that the existence of limit cycles enables successful operation of a turbocharger. This limit cycle is a periodic motion attributed to the nonlinearity of the oil-film, other than the stable and the unstable equilibrium states predicted by the linear analysis. Hence, a nonlinear analysis is required to analyse the limit cycle and to determine the effect of synchronous excitation on it. In the literature a variety of parameters has been shown to influence the dynamic behaviour of a rotor-bearing system. To avoid over-complicated mathematical modeling, the influence of two such parameters: gyroscopic moment and shaft flexibility are first investigated in this thesis using linear stability theory to determine their significance. Effects of gyroscopic action are investigated using symmetric and asymmetric rigid rotors supported on short journal bearings with full-film using rigid and damped supports. In this thesis, the damper supported journal bearing is used to simulate the floating ring bearings that are commonly used in automotive turbochargers. The outer film of the floating ring bearing is treated as an external damper, since the ring is assumed not to rotate but only wobble giving the damping effect from the squeezing action. A gyroscopic coefficient, which is defined as the ratio of the polar to the transverse moment of inertia of the rotor, is introduced. The threshold value of this coefficient is determined to be 1 for the suppression of the conical whirl instability. The stability of the in-phase whirl mode is unaffected by this parameter. A flexible rotor mounted in floating ring bearings with full-film, is analysed to confirm that it behaves as a rigid body up to a speed of 100,000 rpm. Prior to the unbalance response study, a perfectly balanced rigid rotor supported by rigidly supported bearings is first analysed to determine the nonlinear behaviour of the in-phase whirl. To include the stiffness-like radial restoring force, an oscillating 2 π -film cavitation model for the hydrodynamic bearings is used. The effect of a static load on the rotor is analysed to determine the nonlinear behaviour for a wide range of steady-state eccentricity ratios. A parameter plane separating the region of instability from that of stability is presented using linear analysis to determine the stability threshold at which the oil-whirl is initiated. The onset of oil-whirl phenomenon is shown to be the Hopf bifurcation. Particular emphasis is placed on examining the limit cycles (periodic oscillations) around the stability threshold. Reducing the nonlinear equation of motion to Poincare normal form, the first Lyapunov coefficients are evaluated to show the change in the type of bifurcation from sub-critical bifurcation (disappearance of an unstable limit cycle) to super-critical bifurcation (appearance of a stable limit cycle) around the stability threshold. Such bifurcations are demonstrated through plots of orbits using numerical integration by the Runge-Kutta method. With some unbalance added to the rotor-system, waterfall plots are generated to simulate the response characteristics observed in the test data, by running-up the speed. After the Centre Manifold reduction, the equations of motions are averaged for analysis. Using a numerical and an analytical procedure, it is shown that the unbalance is more effective in the transient motion than in the steady-state condition. Unbalance introduces a reduction in the growth rate of whirl amplitude upto a certain optimum unbalance value, above which the effect is reversed. The mechanism behind this behaviour is shown to be the shift in phase caused by the unbalance at the start of whirling, when the dynamic forces are comparable with the unbalance force. This is due to the coupling effect of amplitude and phase in an unbalanced rotor system

No description available.

Additional noise sources are generated when an aircraft engine is mounted beneath a wing. The two main installation sources include: (1) reflection of the exhaust jet mixing noise from the underside of the wing, and (2) interaction between the turbulent jet plume and the trailing edge of the wing, or deployed flap. The strength, directivity and frequency content of these particular sources all serve to increase the time-averaged flyover aircraft noise level heard on the ground by residents beneath the flight path. As the bypass ratio and nacelle diameter of modern turbofan engines continues to increase, constraints on ground clearance are forcing under-wing-mounted engines to be coupled more closely to the wing and flap system, which, in turn, serves to accentuate both of these noise sources. Close-coupled nacelle-airframe designs are now a critical issue surrounding efforts to meet the future environmental targets for quieter civil aircraft. This research is principally aimed at understanding and predicting the groundpropagating noise generated by the latter of these two installed jet noise sources. In order to characterise the jet-surface interaction noise source, however, it is first necessary to isolate it. A small 1/50th model-scale acoustic experiment, therefore, is conducted in a semi-anechoic university laboratory using a single stream jet installed beneath a flat plate. Both far-field acoustic and near-field plate surface pressure data are measured to investigate the jet-surface interaction noise source. Results from this fundamental experiment are then used to help drive a larger, and more realistic, 1/10th modelscale test campaign, at QinetiQ's Noise Test Facility, where 3D wing geometry effects, Reynolds number scaling effects and static-to-flight effects are investigated. A jet-flap impingement tonal noise phenomenon is also identified and investigated at particularly closely-coupled jet-wing configurations. Finally, the first version of a fast, semi-empirical engineering tool is developed to predict the additional noise caused by jet-wing interaction noise, under static ambient flow conditions. It is hoped that this tool will serve to inform future commercial aircraft design decisions and, thus, will help to protect the acoustic environment of residents living beneath flight paths.

Aircraft noise has a negative environmental impact. One of the ways in which it can be mitigated is by placing acoustic liners inside the aircraft's engines. These liners can be optimised for noise reduction. A cost effective way to optimise acoustic liners is to make use of numerical modelling. However, there is room for improvement of the efficiency of current modelling methods. This thesis is concerned with the efficient numerical prediction of noise emitted from modern aircraft engines. Four high order finite element methods are used to solve the convected wave equation, and their performances are compared. The benefit of using the hierarchic Lobatto finite element method to solve this type of problem is demonstrated. A scheme which optimises the efficiency of the high order method is developed. The scheme automatically chooses the most efficient order for a given element, depending on the element size, and the problem parameters on that element. The computational cost of using the standard quadratic finite element method to solve a typical engine intake noise problem, is compared to the cost of the proposed adaptive-order method. A significant improvement in terms of efficiency is demonstrated when using the proposed method over the standard method. Furthermore, a new formulation based on potential flow theory for the solution of vortex sheet problems (typically encountered when dealing with exhaust noise problems) is presented.